In a typical hydraulic fracturing treatment, a fracturing fluid containing a solid proppant is pumped into a formation penetrated by a wellbore at a sufficient pressure to cause the formation or enlargement of fractures in the reservoir. Typically, the subterranean formation has a plurality of production zones. During production of fluids from the well, it usually is desirable to establish communication with selected zones such that stimulation treatments do not inadvertently flow into a non-productive zone or a zone of diminished interest. Selective stimulation becomes pronounced as the life of the well declines and productivity of the well decreases.
Fractures radiate outwardly from the wellbore, typically from a few meters to hundreds of meters, and extend the surface area from which oil or gas drains into the well. The proppant is deposited in the fracture and prevents closure of the fracture. A conductive channel is then formed which extends from the wellbore into the formation being treated.
For certain formations, such as low permeability or “tight” formations (exemplified by gas shale reservoirs exhibiting complex natural fracture networks), wells are drilled horizontally and then subjected to one or more fracture treatments to stimulate production. Typically, when a horizontal well is fractured, the well is divided into several intervals exhibiting different permeability and each interval is stimulated independently.
Subterranean formations typically consist of solid rock fragments and the pore volume in between the rock fragments. The pore volume is generally filled with water, oil and/or gas. Due to density differences, hydrocarbons accumulate in the upper part of the reservoir and water occupies the pores paces below the hydrocarbon zone. During the production of hydrocarbons, water normally displaces the hydrocarbon. As oil wells mature, produced fluids contain greater volumes of water. At some point, the amount of water becomes too high and the production becomes non-profitable. At that point, the one zone is isolated and production within the next zone commences.
Reservoir Monitoring refers to the gathering and analysis of information from reservoirs during production. Such monitoring is used to assess the productivity of zones or producing formations from which fluids are being produced. Monitoring of produced fluids is important in order to increase efficiency of the fracturing operation. Reservoir Monitoring is further used to determine water saturation levels in the well.
In the past, methods of monitoring of produced fluids have used tracers which have been placed in packs in strategic areas within the well. See, for instance, U.S. Pat. Nos. 3,991,827; 4,008,763; 5,892,147 are U.S. Pat. No. 7,560,690. In addition to being restricted to near-wellbore production activity, the monitoring methods of the prior art have a short lifetime following placement of the tracer within the well. Further, the methods of the prior art are unacceptable for use in open hole wells, cumbersome and not particularly cost effective. In addition, the methods of the prior art do not provide a means for controlling the release of the tracer within the well. Alternatives have therefore been sought.
Methods of monitoring produced fluids in other well treatment applications, such as sand control, frac-packing and gravel packing are also desired.